Nutating traction drives of the type wherein a case surrounds and rotatably mounts an outer rotatable structure which has a cylindrical cavity therein with the axis of the cavity being at a non-zero angle from the rotational axis of the rotatable structure and a cylindrical nutatable member nutating within the cavity with the nutatable member having a central cylindrical void in which there is a conically sufaced roller which is in frictional contact with the walls defining the void, are well known. In such a system an output shaft proceeds from the conically surfaced roller and is generally coaxial with the axis of the rotatable structure. For such structures to operate efficiently, it is necessary that the nutatable member be restrained from rotating while it nutates. The prior art attains this restriction from rotation of the nutatable member generally by providing a rigid ground member which is attached to the nutatable member adjacent one end thereof. The ground member has integrally attached thereto a ball or other structure which is constrained within a single slot mounted to the case containing the entire drive. The slot is axially extending and, hence, the nutatable member can nutate since the ball will slide axially during this movement, but, the nutatable member cannot rotate since the slot encloses the ball and completely prevents rotation thereof. The nutatable member overhangs beyond the rotatable member and this overhanging plus the weight of the ground member and friction forces unevenly developed by the interaction between the ball and slot, creates rotational unbalance in the overall structure.
One typical nutating drive of the prior art is found in U.S. Pat. No. 3,224,300 issued Dec. 21, 1965 to Walter V. Chery.